(1) Field of the Invention
The present invention generally relates to a cordless phone having a plurality of personal stations, and more particularly to a cordless phone having a plurality of personal stations and a base station provided with a radio unit communicating with the plurality of personal stations, in which a general calling of the plurality of personal stations can be performed.
(2) Description of Related Art
As more functions are provided for cordless phones, a cordless phone in which a plurality of personal stations can be used is desired. In general, in a case where a cordless phone is provided with a plurality of personal station, the base station must be provided with a plurality of radio units respectively communicating with the plurality of personal stations. In this case, the cost of the cordless phone increases. Thus, it is possible that a plurality of the personal stations share one radio unit in the base station. To prevent that each personal station be determined as a free channel and would be interrupted from other cordless phones, each of the personal stations outputs a carrier signal used for transmission of signals (e.g. a hook signal indicating an on/off-hook operation). In this case, the base station cannot simultaneously receive carriers output from each of the plurality of personal stations. Thus, a time sharing transmission of the carriers has to be performed in a radio channel between the base station and the plurality of personal stations.
A conventional cordless phone provided with a plurality of personal stations will be described below.
FIG. 1 shows a conventional cordless phone having, for example, two personal stations. That is, a base station BS is connected to a line L, and a first personal station PS-1 and a second personal station PS-2 communicate with the base station BS by radio. Operations of the cordless phone in a case of a call-in are shown in FIG. 3. Referring to FIG. 3, a ringing tone signal (a) is activated at predetermined intervals. The base station BS controls output timings of carrier signals from the first and second personal stations PS-1 and PS-2. That is, the base station BS transmits a carrier-on signal to the first personal station PS-1 via a control channel (b). When the first personal terminal PS-1 receives the carrier-on signal, the first personal terminal PS-1 starts outputting a carrier signal (d). Then, the base station receives the carrier signal (d) returned from the first personal station PS-1, a carrier-off signal is transmitted from the base station BS to the first personal station PS-1. When the first personal station PS-1 receives the carrier-off signal, the first personal station PS-1 stops outputting the carrier signal. Then the first personal station PS-1 switches to a service channel identified by the base station BS and becomes in a receive state. After the transmission of the carrier-off signal to the first personal station TS-1, the base station BS transmits a carrier-on signal to the second personal station PS-2 (c). When the second personal station PS-2 receives the carrier-on signal, the second personal station PS-2 starts outputting a carrier signal (d). The base station BS receives the carrier signal returned from the second personal station PS-2, and a carrier-off signal is transmitted from the base station to the second personal station PS-2. When the second personal station PS-2 receives the carrier-off signal, the second personal station PS-2 stops outputting the carrier signal. After this, the base station repeats a transmission of the carrier-on signal and the carrier-off signal to the first personal station PS-1 and a transmission of the carrier-on signal and the carrier-off signal to the second personal station PS-2, in the same manner as that described above. As a result, the first and second personal stations PS-1 and PS-2 alternately output the carrier signals. That is, the time sharing transmission of the carrier signals is performed in the control channel between the base station and first and second personal stations PS-1 and PS-2.
The base station BS checks whether or not the ringing tone signal is activated when the carrier-on signal is transmitted to a personal station. When the base station BS detects that the ringing tone signal is activated, the base station BS transmits a ringing-on signal to the personal station (PS-1 or PS-2) with the carrier-on (e or f). When the personal station receives the ringing-on signal, the personal station starts outputting a ringing tone (g or h). When, after outputting the ringing-on signal, the base station BS detects that the ringing tone signal is not activated, the base station BS transmits a ringing-off signal to the personal station. When the personal station receives the ringing-off signal, the personal station stops outputting the ringing tone.
The ringing-on signal and the ringing-off signal described above are included in a telegraphic message used for transmitting the carrier-on signal. The telegraphic message has a format, for example, as shown in FIG. 2. That is, the telegraphic message is formed of a synchronous signal (SYNC.), an identification code identifying a radio channel formed between the base station BS and the personal stations, a sub-identification code (SUB ID) identifying a personal station (PS-1 or PS-2) to which the telegraphic message should be transmitted, the carrier-on signal and the ringing-on/off signal.
Due to the above processes, the first and second personal stations PS-1 and PS-2 output the ringing tones. That is, a general calling of the first and second personal stations PS-1 and PS-2 is performed. Under a condition in which the ringing tones are being output from the first and second personal stations PS-1 and PS-2, when, for example, an off-hook operation of the second personal station is carried out, the second personal station PS-2 activates a hook signal (i) and the activated hook signal is transmitted to the base station BS. The base station receives the activated hook signal transmitted from the second personal station PS-2 and transmits a command to stop outputting the ringing tone to the first personal station PS-1. When the first personal station receives the command, the first personal station stops outputting the ringing tone. After the off-hook operation, the second personal station PS-2 continuously outputs the carrier signal(j). A communication between the second personal station PS-2 and the base station BS is performed via the service channel, so that the second personal station PS-2 can communicate with the external station via the base station BS.
In the above conventional cordless phone having the plurality of personal stations, as the base station controls output timings of the carries from the personal stations, the base station must determine what number of personal stations are operatively coupled thereto. In addition, the base station must transmit the carrier-on signal and the carrier-off signal alternately to every personal station. Thus, control processing in the base station is complex. Further, the carrier-on signals and the carrier-off signals for a plurality of personal stations are output from the base station at times different from each other, so that the ringing tones are not simultaneously output from the plurality of personal stations, as shown in (g) and (h) of FIG. 3. Thus, the ringing tones output from the plurality of personal stations are unpleasant.